1. Field of the Invention
This invention relates to space heating and ventilation systems for buildings, and concerns a heating and ventilation unit for such a system.
2. The Prior Art
Hot air ventilation systems for buildings are well known in many forms, the usual form being that in which fresh air is directed past a heating and ventilation unit which raises the temperature of the air before releasing it into the various rooms of the building, to which it is distributed through suitable ducting. Stale air escapes from the building to the atmosphere outside through extractor fans, ventilators (such as air bricks) or both, and through cracks around windows and doors.
The air is customarily heated either by combustion of fuel, which may be gas or oil or solid fuel, or by electricity. In the case of heating and ventilation units employing the combustion of fossil fuels, the hot and potentially toxic products of combustion are not allowed to mingle with the fresh air being heated, but instead are vented to the atmosphere after giving up some of their heat by indirect transfer to the fresh air. Previously, this has been achieved by a static heat exchanger located in the fresh air flow. However, the heat exchangers used hitherto have been thermally inefficient in relative terms, and attempts to improve their efficiency (for example, by the use of extended surface heat transfer elements) have met problems of increased noise and have required more powerful fans. The use of electricity for heating the fresh air is acceptable environmentally, but inherently expensive.
In order to improve the thermal efficiency of hot air ventilation systems it has been proposed to use the heated stale air being extracted from the building to heat the fresh air entering it. In a known systems the two gas flows are constrained to pass through a static plate heat exchanger generally in cross-flow mode. However, the thermal efficiencies of place heat exchangers are relatively low, especially if, as in these applications, the fluids being handled are gaseous and their difference in temperature is not large. In order to achieve an acceptable thermal efficiency for the whole hot air ventilation system, therefore, some recirculation of extracted air back into the building has hitherto been considered necessary. However, such recirculation of air carries a resultant health risk arising from the possible spread of air-borne bacteria and viral infections throughout the building.
A further form of energy recuperation which has been proposed for hot air ventilation systems in commercial and industrial buildings is a rotary heat exchanger through which the stale and fresh air flows are caused to pass in opposite directions. In this case the thermal transfer elements are formed of superimposed layers of alternately flat and corrugated aluminium foil forming a multiplicity of gas passages which extend axially of the heat exchanger. However, these heat exchangers have an operating temperature limit of 70.degree. C. and require an extended multistage process to provide a hygroscopic coating for applications where total heat (rather than solely sensible heat) is to be transferred. A somewhat similar coating process enables the heat exchangers to be used for sensible heat (only) applications at temperatures up to 350.degree. C. In either case, the rotary heat exchanger is not capable of withstanding the temperatures of 600.degree. C. or more which are typical of the (uncooled) gases produced by fossil fuel combustion.
From the foregoing it will be understood that the use of fossil fuel heating together with heat recuperation in hot air ventilation systems has hitherto generally required two heat exchangers, one for transferring heat from the combustion gases to the fresh air, and the other for preheating the fresh air with the stale air from the space being heated and ventilated. It has further been proposed, however, that by use of a temperature resistant material (e.g. a ceramic), that a single rotary heat exchanger can be used to perform both the heating and recuperation functions. Two such arrangements are particularly described with reference to FIGS. 2 and 3 of GB Patent Publication No. 2143027A. Each of these arrangements has a rotary heat exchanger formed of two concentric discs through the radially outer one of which the majority of the stale air is passed and through the radially inner one of which the remainder of the stale air is passed after heating by, for example, natural gas. The discs are stated to be preferably ceramic but their composition is not specified; moreover, it is suggested that they should be formed by winding alternate flat and corrugated sheets, presumably in spiral manner.
The fresh air to be heated is passed through the discs of the heat exchanger of Publication No. 2143027A in contraflow mode to the stale air, so as with rotation of the heat exchanger to extract from the heat exchanger the heat supplied by the stale air and the gas burner. Headers are provided for segregation of the individual flows of gas from one another on each side of the heat exchanger, but it is not specified in what proportions these headers divide each disc between the stale and fresh air flows, nor is there any provision described for ensuring that the possibly toxic combustion products of the burner will not be carried over by the heat wheel and so mingle with the fresh air entering the building. Thus, whilst Publication 2143027A generally describes a hot air ventilation system which employs a rotary heat exchanger of a ceramic material arranged both for heating the incoming fresh air by means of a fossil fuel burner and for recuperating heat from the exiting stale air, the disclosure of Publication 2143027A is lacking in certain important details; moreover, Applicants believe that by confining the hot gases to only a minor portion of the heat exchanger, the heat exchanger is underutilised thermally and is correspondingly bulky and expensive to produce.